Semiconductor Optical Amplifiers (SOA) are essential components in optical networks. Besides acting as optical amplifiers, their inherent non-linearities allow them to form the basis of many signal processing elements (e.g. wavelength converters, logic gates). Compared to the Erbium Doped fibre amplifier (EDFA), Erbium Doped waveguide amplifier (EDWA) or Raman amplifier, the SOA has many advantages: lower cost, larger bandwidth, smaller size, and the potential to be integrated on a chip with electrical pumping. However SOAs suffer from the disadvantage that they have a higher noise figure than EDFA or EDWA amplifiers.
Attempts have been made to reduce the noise figure of SOAs. One solution involved embedding a lasing cavity inside the SOA. However, this arrangement resulted in a significant reduction of gain because at a certain level of bias current, the SOA starts lasing, this results in clamping of the carrier density at a specific value corresponding to when the gain equals the cavity losses.
Therefore there is a need for a semiconductor optical amplifier with a reduced noise figure and a relatively high gain.